I have also failed installing security certificates on my N9. Certificate manager finds the packages, ask correctly for password for import, then gives me a dialog for use settings. I can not see "Done" button, only back arrow. And I still get "Install" button under the certificate screen, though my certificates show in the installed list.
Anyway when I navigate the browser to web site that uses my certificates I always get notification for it being untrusted(although this is my bank, using Verisgn) and I am neither prompted to select my personal certificate nor it is used automatically. So I can not perform online banking operations with default browser.
The exact same certificates I was able to import in Mozilla and I am using them without any problems. But I prefer the default browser or Opera than the sluggish Mozilla...
Anyone managed to solve this?